1. Field of the Invention
The invention relates to a method for calibrating a photographic copy printer.
2. State of the Art
Numerous copying methods are known. Most often, this involves the picture fields of a negative film being exposed to light and copied to photo paper. Hereinafter, for the sake of simplicity, reference will be made to negative films and photo paper. The negative films or picture fields on the negative films are representative of original material or, in general, originals to be copied (including slides), and the photo paper or paper pictures are representative of the copy material or copies (including, sheets).
Because normally standardized, uniform amounts of copying light cannot be practically used in copying equipment, the picture fields of the negative fields of the negative films are analyzed. Typically, this is done such that in a measuring station of the copy printer, first the picture field of the negative film to be copied is scanned regionally, (for example, dotwise) with the aid of measurement light. The measurement light transmitted or remitted from each scanned region of a picture field of the negative film is transmitted to a detector array, broken down spectrally, and converted into wavelength- and intensity-dependent electrical measurement signals. The electrical measurement signals are then digitized, and with their aid, picture field-specific measurement data are ascertained. With the aid of the measurement data acquired, requisite amounts of copying light are then ascertained.
For the requisite amounts of copying light in the various colors (blue, green, red) to be ascertained correctly, the manner in which a certain photo paper responds to changes in the various colors of the copying light must be known. That is, changes in the paper density (via the concentration of the pigments yellow, magenta and cyan) as a function of a change in the copying light in the colors blue, green, red must be known. A model for the photo paper--a paper model--must therefore be known which is representative for the absorption properties of the photo paper upon a change in the exposure (including the secondary absorptions of the photo paper; for example, if the photo paper is exposed to blue light, this causes a change in the optical densities in all three colors).
In addition to knowing a paper model, the manner in which the photo paper reacts to changes in the spectral film density must be known. That is, the manner in which the copying light (exposure) has to be changed to compensate for deviations in the actually measured density of a negative from the average film density (or in other words the deviation from the "standard") must be known. Because the film must be viewed "with the eyes of the photo paper", this involves a paper-related film model.
Once the paper-related film model has been optimally setup-tested, all negative picture fields can be copied to a uniform gray with the aid of this model and with the aid of the measurement data of the respective negative picture field. This copying can be achieved with great accuracy, in the case of negative picture fields in which each one has been exposed homogeneously per se but each negative picture field has a different density from the others. Such accuracy is possible, regardless of the particular film type, and regardless of the particular density of the individual negative picture fields. With the aid of the paper model, once it has been tested, it is possible to predict what change in the copying light in the particular color (blue, green, red) is needed to achieve a certain change in paper density (in yellow, magenta, cyan).
With a copy printer which, in operation, is setup-tested as far as possible (that is, calibrated) with the aid of the ascertained measurement data of the respective negative picture field, and with the aid of the paper-related film model, the amounts of copying light required for the incident exposure are ascertained. The paper model or the inverse paper model is used only in the case of a second pass; namely, whenever the corrections entered by the operating staff have to be converted to exposure corrections. On the basis of the amounts of copying light ascertained, corresponding control signals are ascertained for color filters and shutters provided in the copying station of the copy printer which, in accordance with the control signals, are placed in the copying beam path when the negative picture field is copied onto the photo paper. Once the copying of the negative picture field onto the photo paper has been completed, the copies produced are developed in a developing station which need not necessarily, but which may be, an integral component of the copy printer.
The above-described method is functionally reliable and performs well. However, with the method described above, the setup test or calibration of the copy printer is done using quite a large number of special test negative picture fields. Although these special test negative picture fields are indeed exposed homogeneously, (taken each on its own), they are exposed differently in comparison with the other test negative picture fields. Many different film types are used, since the various film types have different spectral density courses, which must be balanced out by the copy printer in copying. These different test negative picture fields are copied onto photo paper before the production process begins, and then are developed and finally measured by densitometry. From the developed test copies, the operating staff can then determine whether the copy printer has been optimally setup-tested (that is, calibrated). If this is not the case, then the calibration is corrected, on the basis of the measured data of the paper and film, and on the basis of the memorized exposure values. For example, for subtesting of the paper-related film model, a test roll of film is used, with about 10 different film types and 100 negative picture fields.
While this expenditure is still acceptable for a high capacity printer, such effort and expense are no longer acceptable for setup testing in minilabs. It is also disadvantageous that once the copy printer has been setup tested (the first thing every day, for instance), no ongoing monitoring of the copy printer settings (and hence the settings for the two models) is performed. At best, the operating staff performs sporadic monitoring by making random checks of the production paper pictures or by regularly producing and assessing test pictures.